Display devices, such as liquid crystal display devices, include video signal line driver circuits (also referred to as “source drivers”) for generating gradation voltages on the basis of gradation reference voltages, selecting voltages from the generated gradation voltages in accordance with video signals, and applying the selected voltages to video signal lines (also referred to as “source lines”) as analog video signals, and such display devices also include scanning signal line driver circuits (also referred to as “gate drivers”) for sequentially applying high-level scanning signals to scanning signal lines (also referred to as “gate lines”) in order to sequentially activate the gate lines and thereby write the analog video signals applied to the source lines in pixels.
In the case of such a display device, to generate analog video signals corresponding to externally inputted digital video signals, the source driver selects some gradation voltages, which are generated on the basis of gradation reference voltages by means of a voltage ladder provided as a gradation voltage generation circuit, and as the analog video signals, the source driver applies the gradation voltages to a plurality of source lines formed in a display panel. In this case, gradation voltages of the same value might be simultaneously selected for a number of source lines. In this manner, if such gradation voltages of the same value are simultaneously selected for a number of source lines, a higher current flow through resistive elements that should output the gradation voltages in the voltage ladder. As a result, due to voltage drop, the voltage ladder might output gradation voltages with values lower than values with which the gradation voltages should originally be outputted.
FIG. 9 is a diagram illustrating a video displayed on a display panel where, due to voltage drop, values of gradation voltages are lower than values with which the gradation voltages should originally be applied. As shown in FIG. 9, black areas 101 are displayed in upper left and upper right portions of a screen, and between these areas, there lies an area 102 displayed in white. Moreover, displayed across a lower portion of the screen is a white area 103. On such a screen, one horizontal line next to another horizontal line indicating the bottom of the black areas 101 should originally be displayed in white, but a black line 104 might be displayed, as shown in FIG. 9.
The reason why such a black line 104 is displayed will be described. FIG. 10 is a diagram illustrating the configuration of a conventional source driver 150 described in Patent Document 1. Analog video signals outputted by the source driver 150 are signals selected in accordance with an inputted video signal, from among gradation voltages derived from output terminals of a voltage ladder 87 provided in the source driver 150. Accordingly, when a video displayed on a horizontal line is changed from a combination of black and white to simply white, the voltage ladder 87 should simultaneously output simply a number of gradation voltages corresponding to the white video. In this case, of all gradation voltages outputted by the voltage ladder 87, the gradation voltages corresponding to the white video are selected a number of times. As a result, the current that is required for outputting such gradation voltages is limited, whereby the output voltage of the source driver 150 decreases, so that voltages with values lower than values that the voltages should originally have are applied to source lines. Therefore, on the display panel, the horizontal line that should originally be displayed in white is reduced in luminance, and the horizontal line with the reduced luminance is seen as a black line.
In particular, in the case of a display panel with a high-definition resolution called “4K” or “8K”, the number of gate lines is very high, respectively approximately 2000 or approximately 4000, and therefore, the drive frequency of the source driver 150 is high. Accordingly, before the gradation voltages outputted by the voltage ladder 87 are recovered from reduced values to original values, the source driver 150 has to output gradation voltages for the next horizontal line, and therefore, the black line is more likely to be seen.
Patent Document 1 discloses a display device including the source driver which, as shown in FIG. 10, is provided with charge supplement circuits 58 for providing charge supplement to respective gradation voltage lines 98 through which gradation voltages generated by the voltage ladder 87 are outputted, whereby even when voltage drop occurs to the gradation voltage lines 98 due to, for example, the timing of switching video data, the gradation voltages can be stably outputted by promptly recovering the gradation voltages to original values.